Multiple-step process of fiber liberation



' duce a pulp which at thetime of liberation components, inc udingpentosans, and which' first .liquor employed is a SuIphurousacid- HoDrawing.

ent invention, three chemical liquors of difmaterial and in so modifyingthe constitution Patented Jan. 19,1932

UNITED STATES PATENT OFFICE enonen a. Brennan, or nnanm, rmw nssronoa'ro :Bnowrr CODA-NY, or

- BERLIN, NEW a coarom'rron or we v111ml:Iran-swan rn'ocnss or FIBERmanner I museum .153? e, 1980. Serial I'm 450,314.

This invention relatestothe liberation of necessary to complete fiberliberation, but

fiber from raw cellulosic materials to proalso a higher yieldof pulp,basedon wood, {without impairing thebleachability of the is practicallyfree from non-alpha cellulose pulp. The intermediate treatment with thebisulphite liquor is efiected under conditions designed not to isolatethe fiber content of thecellulosic material. The combined S0 contentdes'ired in'the bisulphite liquor may is characterized by a lowso-called solution viscosity, which renders it especially valuable forconversion into derivatives to be treated with solvents to formsolutions of low. viscosity.

- soda, sodium sulphite, or sodium sulphate to In accordance with theprocess of the presthe sulphurous acid solution remaining after thefirst treatment. These chemicals may be ferent compositions are employedin the treatment of the cellulosic material before its fiber content isisolated as pulp freed from the associated organic materials. Eachliquor exerts a chemical action on'the cellulosic material, and thuscontributes to the results secured, but there is a preferable se quencein which the liquors are used. The

in amount calcillated to produce the desired combined SO content. Whensuch aproce- [dure is followed, heat consumed in raising the temperatureof the sulphurous acid solution of the first treatment is conserved forthe secof the first treating solution is also conserved used in thesecond treatmentyand the time lost when the first treating solution isdrained 'from the chips and replaced by a previously prepared bisulphitesolution is avoided. 1 g The process of the present invention isapplicable to various raw cellulosic materials solution which isespecially effective for removing pentosans from the raw cellulosic ofthe cellulose fibers asto lead to a final product of low solutionviscosity. The s ec 1 0nd liquor is a bisulphite solution which may beaddedto the cellulosic material after the sulphurous acid solution hasbeen drained therefrom, or which may be prepared from the sulphurousacid solution by adding suitable chemicals thereto. The bisulphitesolution is an active agency in the removal of ligneous matter containedin the cellulosic materialand in conditioning the cellulosic materialfor the treatment with the third chemical liquor, which brings aboutcomplete fiber liberation. The third chemical liquor is a solution ofalkali, such as that used in the soda or kraft process, which; even atlow concentrations, readily completes the ,removal of residual ligneousmatter contained in the cellulosic material to effect complete fiberliberation. I f

The treatment of the cellulosic material with a bisulphite liquorbetween the first treatment with a sulphurous acid solution and thetreatment with a solution of alkali is advantageous, in that it not onlymakes possible a reduction in the quantity of alkali in material such ashardwoods, hemp, bamboo, and bagasse. It is also applicable to wood suchas spruce, which is" an excellent 1 raw material for the production ofpulps of high alpha cellulose content and suitable both for papermakingand for the preparation of cellulose derivatives. A specific example ofprocedure which may be practised with eminently successful results inthe case of spruce, may be substantially as follows. The chipped wood,which may be in the form of. chips of the normal size, or of a sizelarger or smaller than that usually employed in a pulp mill, is firsttreated with a sulphurous acid solution containing about 2% to 5% S0This treatmaintained for about two hours while the pressure isordinarily not allowed to exceed ondtreatment. The sulphur dioxidecontent ment may be efiected in adigester in which. the charge-of woodand liquor may be confined under pressure and at elevated temperature.The digester contents are heated to atemperature of 230 to 260 F whichis be obtained by adding alkali, such as caustic added in the form ofconcentrated solutions 7 for the preparation of the bisulphite solutionwhich a-re of high pentosan content as found 7 about to 75 pounds; butin special cases higher pressure may be reached. At the end of thistime, the solution may be drained from the chips and a suitablebisulphite liquor added to the digester, but it is preferable to formthe bisulphite liquor from the sulphurous acid solution of the firsttreatment. Ac-

cordingly, a concentrated solution of caustic soda, sodium sulphite, orsodium sulphate may be added to the sulphurous acid solution until thedesired combined SO content is produced therein, say, 1% to 2%, orgreater. When sodium sulphate is the added chemical, only a relativelysmall portion will react with the sulphurous acid solution to formbisulphite, but as the sulphite present in the liquor is consumed byreaction on the cellulosic material, more bisulphite is formed from theexcess sulphate present in the liquor. By injecting the added chemicalsin the sulphurous acid solution in the digester, the liquor and chipsmay be held at a temperature of 250 to 260 F., under which temperatureconditions it is desired to effect the treatment with the resultingbisulphite liquor. If the combined SO content of the liquor is greaterthan, say, 1% to 2%, the temperature of cooking may be advantageouslyraised to, say, 285 to 300 F. The treatment with the bisulphite liquormay be for one to two hours, at the end of which time examination of thecellulosic material indicates that it is still in the form of firmchips, even though some softening has taken place. The bisulphite liquoris then withdrawn from the digester and the chips are preferably washedin order to avoid loss of alkali from the alkaline cooking liquor byreaction with free acid carried by the chips.

The washed or unwashed chips are then digested in an alkaline liquor thealkalinity of which is afforded by the hydroxide, sulphide, or sulphiteof sodium, or by a combination of these chemicals. The alkaline liquorneed contain only about one pound of active alkali per cubic foot,calculated as Na O, to comlete fiber liberation, suchan alkalinity bemgabout half that required in the alkaline liquor used for the productionof kraft pulp. Preferably, the liquor contains alkali in the form ofcaustic soda and sodium sulphide,

temperature and pressure conditions used in the kraft process, say, 300to 335 F. and 67 to 110 pounds pressure, it is found that comlete fiberliberation is effected in about two ours. The resulting pulp, afterbeing washed free of spent cooking liquor and analyzed, shows an alphacellulose content of 96% to 98%, a pentosan content of less than 1%, anda viscosity of about 1." Such a product compares favorably with woodpulps which have been refined by subjecting chemical wood pulps such assulphite or kraft, to chemical refining treatments; and in fact it is ofhigher alpha cellulose but of lower pentosan content and has a lowersolution viscosity. The solution viscosity value of the pulp isconsiderably lower than that of the usual commercial wood pulps, such assulphite or kraft, or refined pulps prepared therefrom. The raw pulpobtained by a process such as described may be bleached to whiteness inthe usual bleach liquors, but it is preferable to employ a bleach liquorcontaining sufiicient free alkali to maintain a condition of distinctalkalinity in the liquor throughout the bleaching operation, in order topreserve the alpha cellulose content of the pulp while coloring mattersare being removed.

The broad principles of the process hereinbefore described may beapplied by treat ing the raw cellulosic material first with thebisulphite liquor and then with the sulphurous acid solution. Thebisulphite treatment, if practised under the conditions previouslydescribed, will not be accompanied by a substantial isolation of fiber,but the subsequent treatment should be carried out in a sulphurous acidsolution containin only about 1% to 2% S0 in order to avoi substantialfiber liberation. Such a sequence of ste s does not, however, permit theuse of the liquor together with its heat and chemicalsin the secondtreatment, as is the case when a sulphurous acid solution is employed in{the initial treatment.

It is important that the completion of fiber liberation during the firsttwo treatments be avoidedas much as possible in producing the product ofthe present invention, as the action'of an acid solution, particularlyof a hot sulphurous acid solution, on a liberated pulp is to injure thepulp seriously for pa makingpurposes, as well asto diminish gh e yieldof product obtained after digestion in the alkaline liquor. Bypermitting the acid solution to act on the cellulosic material while itis associated with non-fibrous organic ma.- terial on which the acidsolution may exert a selective chemical action, the fiber content 3 ofthe cellulosic material may be preserved to a considerably greaterdegree.

, I claim:

1. A multiple-step process of fiber liberation, in which the fibercontent of raw cellulosic material is liberated in the last step, whichcomprises subjecting the raw oelluloslc material to the independentaction of a sulphurous acid solution and of a bisulphite solution, andthen cooking the material so treated in an alkaline liquor to'completefiber liberation.

2. A multiple-step process of-fiber liberation,'which comprises cookingraw cellulosic material successively in a sulphurous acid solution, abisulphite solution, and an alkaline liquor, while keeping such materialshort of complete fiber liberation until the alkaline liquor exerts itsaction thereon.

3. A process of fiber liberation, which comprises cooking raw cellulosicmaterial in a, sulphurous acid solution, converting the hot sulphurousacid solution into'a bisulphite liquor, cooking in the bisulphite liquorshort of fiber liberation, and finally cooking in an alkaline liquor tocomplete fiber liberation.

In testimony whereof I have afiixed my signature. I

GEORGE A. RICHTER.

